Ilmenite, altered ilmenites and rutile are the major, commercially-important mineral feedstocks for titanium metal and titanium dioxide pigment production. Most of the world's mined ilmenite is used for the production of titanium dioxide pigments for use in the paint and paper industries. Pigment grade TiO.sub.2 has been traditionally produced by reacting ilmenite with concentrated sulphuric acid and subsequent processing to produce a TiO.sub.2 pigment--the so-called sulphate route. However this process is becoming increasingly unacceptable on environmental grounds due to the large volumes of acidic liquid wastes which it produces. The alternative process--the so-called chloride route--involves reaction with chlorine to produce volatile titanium tetrachloride and subsequent oxidation to TiO.sub.2. Unlike the sulphate route, the chloride route is capable of handling feedstocks, such as rutile, which are high in TiO.sub.2 content and low in iron and other impurities.
Consequently the chloride-route presents fewer environmental problems and has become the preferred method for TiO.sub.2 pigment production. Also, whilst the sulphate route is capable of producing only TiO.sub.2 pigments, both titanium metal and TiO.sub.2 pigments can be produced via the chloride route. Natural rutile supplies are insufficient to meet the world demands of the chloride-route process. Thus there is an increasing need to convert or upgrade the more-plentiful ilmenites and altered ilmenites (typically 45 go 62% TiO.sub.2) to synthetic rutile (containing over 90% TiO.sub.2).
Several processes are known for the production of synthetic rutile. The most commonly-practised process, usually referred to as the Becher process, involves the following main stages:
1. Reduction, in a rotary kiln, of the iron oxides contained in the ilmenite feed largely to metallic iron using coal as the heat source and the reductant, the resultant mixture of metallic iron and titanium phases being known as "reduced ilmenite". PA1 2. Cooling of the solids discharging from the reduction kiln. PA1 3. Dry physical separation of the reduced ilmenite and surplus char. PA1 4. Aqueous oxidation (known as aeration) of the reduced ilmenite to convert the metallic iron to iron oxide particles discrete from the TiO.sub.2 -rich mineral particles. PA1 5. Wet physical separation to remove the iron oxide from the TiO.sub.2 -rich mineral comprising the synthetic rutile product. PA1 6. An optional acid leaching stage to remove a portion of the residual iron and manganese and magnesium. PA1 7. Washing, dewatering and drying of the synthetic rutile product. PA1 1. The addition of a chlorine-containing compound together with sulphur or a sulphur-containing compound, as disclosed in Australian patent No. 516,155, with the object of decreasing the amount of pseudobrookite formed and thereby increasing the amount of iron available for metallisation. PA1 2. The common practice of adding sulphur or a sulphur-containing compound (i.e. no chlorine-containing additive), as described in the aforesaid paper, and with the same object as in (1) above. PA1 3. The addition of a magnesium and/or manganese compound, as disclosed in U.S. Pat. No. 3,502,460, with the object of producing an acid-soluble synthetic rutile product. PA1 4. The addition of a flux in the form of a glass-forming reagent, e.g. a borate salt or mineral such as calcium borate, as disclosed in International patent publication WO94/03647 (PCT/AU93/00381), with the object of assisting in the removal of radioactive elements.
In one advantageous but by no means exclusive application, the present invention is applicable to the first of these stages.
A paper entitled "Synthetic Rutile Operations of RGC Mineral Sands Limited at Capel and Narngulu, WA" ("Australasian Mining and Metallurgy", 1993, pp 1301 to 1304, published by The Australasian Institute of Mining and Metallurgy) describes, inter alia, reduction kiln operation including the feature of feeding coal to both the feed and discharge ends of the kiln.
Several other processes have been proposed for upgrading ilmenite or other ferro-titaniferous ores, and a number of these include a reduction step performed in a kiln.
One general class entails reduction of the iron to the ferrous state, followed by a direct acid leach of the kiln product to remove the iron. The so-called Murso process, described for example in British patent 1225826, involves a pre-oxidation of iron in the feed to the ferric state followed by reduction in a kiln, preferably using a gaseous reductant such as hydrogen, to the ferrous state or to a mixture of the ferrous and metallic states. Another process, known as the "Hybrid" process and disclosed for example in international patent publication WO91/13150, involves control of the temperature in the reduction kiln at a lower value than in the commercial Becher process so as to favour formation of a metatitanate phase which is readily leached to remove impurities.
A number of modifications to the Becher process, involving the addition of various reagents to the reduction kiln, have been proposed and/or are practised. These include the following:
The practice hitherto has been to introduce these various reagents at the feed end of the kiln, that is added in parallel with the ilmenite feed or premixed with the ilmenite feed. The problem with this hitherto method of addition is the poor incorporation of the reagent(s) into the mineral. The word "incorporation" means the valuable portion of the added reagent(s) found to be present in the product such as reduced mineral, e.g. reduced ilmenite, as discharged from the kiln, expressed on a weight for weight basis. For example, using ferrous sulphate as a sulphur-containing reagent, percentage sulphur incorporation is the kg per hour of sulphur equivalent present in the reduced mineral discharged from the kiln (after deducting sulphur contributed by the coal and feed mineral) divided by the kg per hour of sulphur equivalent of the reagent times 100. It will be appreciated that some reagents are incorporated more/less readily than others.
The poor incorporation observed to date is demonstrated where elemental sulphur or ferrous sulphate (a sulphur-containing material) is introduced at the feed end of a reduction kiln. In this case, sulphur incorporation is in the range of only 15 to 30% and, in our experience, most typically 23%. This is not only wasteful in terms of reagent consumption but also leads to other difficulties. For example, that portion of sulphur not incorporated reports as H.sub.2 S, SO.sub.3 and SO.sub.2 in the reduction kiln exhaust gas, incurring expensive gas cleaning costs. In other instances, that portion of reagent(s) not incorporated condenses and leads to build-up in exhaust gas extraction systems.